The present invention relates to an exposure method, a phase shift mask used in the same, and a process for fabricating a semiconductor integrated circuit device using the same. More particularly, the present invention relates to a technology which is effective if applied to an exposure technology using a phase shift mask for imparting a phase difference in an exposing light at an exposing step.
As the miniaturization of a semiconductor integrated circuit so advances that the rules for designing circuit elements or wiring lines come to an order of submicrons, there arises a serious problem of a drop of the pattern transfer precision in the photolithography step of transferring an integrated circuit pattern on a photomask to a semiconductor wafer by the action of a light such as the i-line (having a wavelength of 365 nm).
In case, for example, a pattern composed of transparent areas P and opaque areas N on a photomask 30 (including a reticle), as shown in FIG. 34, is to be transferred onto a semiconductor wafer, the individual lights just having passed through the paired transparent areas P and P across one opaque areas N have an identical phase.
Over the semiconductor wafer, therefore, those two lights interfere and intensify each other at their boundary. In case the opaque areas N have a smaller width than the exposure wavelength, the projected pattern image on the semiconductor wafer has its contrast dropped to reduce its focal depth so that its transfer precision is drastically degraded. This problem becomes the more serious for the finer pattern of the semiconductor integrated circuit.
As means for solving such problem, there has been noted a phase shift technology for preventing the drop of the contrast of the projected image by changing the phases of the lights to pass through the mask.
The phase shift technology has been disclosed in Japanese Patent Publication No. 50811/1987 or Japanese Patent Laid-Open No. 67514/1987, No. 140743/1990 or No. 252659/1991, for example.
First of all, Japanese Patent Publication No. 50811/1987 has disclosed a phase shift technology for weakening the optical intensities of two lights over the semiconductor wafer at their boundary by providing a phase shifter of a transparent film in one of a pair of transparent areas across an opaque area on a photomask so that the two lights having passed through the paired transparent areas may have their phases inverted.
In this phase shift exposure technology, when a pattern composed of transparent areas P and P and an opaque area N on a phase shift mask 31, as shown in FIG. 36, is to be transferred onto the semiconductor wafer, either of the paired transparent areas P and P is equipped with a phase shifter 32 of a transparent film having a predetermined refractive index, and the phase shifter 32 has its film thickness adjusted to invert the phases of the two lights just having passed through those transparent areas P and P. As a result, the two lights interfere and weaken each other at their boundary on the semiconductor wafer, as shown in FIG. 37, so that the pattern has its projected image remarkably improved in contrast to have an excellent transfer precision.
On the other hand, Japanese Patent Laid-Open No. 67514/1987 has disclosed a phase shift technology for improving the transfer precision of a pattern by providing second minute transparent areas around the first transparent areas on the phase shift mask and by equipping one of the transparent areas with a phase shifter for inverting the phases of the two lights having passed through the paired transparent areas.
On the other hand, Japanese Patent Laid-Open No. 140743/1990, as made by us, has disclosed a phase shift exposure technology for improving the pattern transfer precision by equipping a portion, e.g., the outer periphery of one transparent area with a phase shifter for emphasizing the edges to invert the phases of the two lights having passed through the portions having the phase shifter and not, so that the pattern transferred may have its contrasts emphasized between its center and periphery.
Furthermore, Japanese Patent Laid-Open No. 252659/1991 has disclosed a phase shift mask for improving the transfer precision of a pattern by arranging a transparent area having a relatively large width at its center in each of a plurality of transparent areas arranged across an opaque area on a mask substrate and by arranging relatively narrower transparent areas, which give their transmitted lights a phase inverted from that of the lights having transmitted through the wider transparent areas, at the two sides of the wider transparent areas.